[PATCH v2 2/3] blk-cgroup: skip dying blkg in blkcg_activate_policy()

[Zheng Qixing]

From: Zheng Qixing <zhengqixing@xxxxxxxxxx>

When switching IO schedulers on a block device, blkcg_activate_policy()
can race with concurrent blkcg deletion, leading to a use-after-free in
rcu_accelerate_cbs.

T1:                               T2:
		                  blkg_destroy
                 		  kill(&blkg->refcnt) // blkg->refcnt=1->0
				  blkg_release // call_rcu(__blkg_release)
                                  ...
				  blkg_free_workfn
                                  ->pd_free_fn(pd)
elv_iosched_store
elevator_switch
...
iterate blkg list
blkg_get(blkg) // blkg->refcnt=0->1
                                  list_del_init(&blkg->q_node)
blkg_put(pinned_blkg) // blkg->refcnt=1->0
blkg_release // call_rcu again
rcu_accelerate_cbs // uaf

Fix this by replacing blkg_get() with blkg_tryget(), which fails if
the blkg's refcount has already reached zero. If blkg_tryget() fails,
skip processing this blkg since it's already being destroyed.

Link: https://lore.kernel.org/all/20260108014416.3656493-4-zhengqixing@xxxxxxxxxxxxxxx/
Fixes: f1c006f1c685 ("blk-cgroup: synchronize pd_free_fn() from blkg_free_workfn() and blkcg_deactivate_policy()")
Signed-off-by: Zheng Qixing <zhengqixing@xxxxxxxxxx>
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
---
 block/blk-cgroup.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index 600f8c5843ea..5dbc107eec53 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -1622,9 +1622,10 @@ int blkcg_activate_policy(struct gendisk *disk, const struct blkcg_policy *pol)
 			 * GFP_NOWAIT failed.  Free the existing one and
 			 * prealloc for @blkg w/ GFP_KERNEL.
 			 */
+			if (!blkg_tryget(blkg))
+				continue;
 			if (pinned_blkg)
 				blkg_put(pinned_blkg);
-			blkg_get(blkg);
 			pinned_blkg = blkg;
 
 			spin_unlock_irq(&q->queue_lock);
-- 
2.39.2